How to Fix Under-Extrusion in 3D Prints
Diagnose: extrude 100 mm and measure
Heat the hotend to print temp. Mark the filament 120 mm above the extruder. Command 100 mm of extrusion in the printer's manual control. After it finishes, the mark should be 20 mm above the extruder. If less, the extruder slipped. If much more, the printer is over-feeding.
Cause 1: extruder slipping
- Drive gear grub screw loose
- Drive gear teeth packed with filament dust (clean with a brass brush)
- Tension on idler too low
- PTFE tube not seated against the hotend (Bowden)
Cause 2: clogged nozzle
- Cold pull: heat to print temp, push filament through, cool to 90 C, pull out, repeat until clean
- Atomic / Capricorn cold pull works on most clogs
- If cold pulls fail, remove the nozzle and soak in solvent or replace
Cause 3: wet filament
Wet filament foams at the nozzle, creating gaps. Dry the spool and re-test. PLA dried at 50 C for 4 hours, PETG at 65 C, nylon at 80 C for 8 hours.
Cause 4: print temp too low for speed
Speed and temp are linked. If you print fast, you need more heat to melt the filament in time. Raise nozzle temp 5 to 10 C if printing above 100 mm/s.
Cause 5: flow rate
Only adjust flow rate (extrusion multiplier) after eliminating mechanical and clog issues. Print a single-wall calibration cube and measure the wall thickness; adjust flow to match the slicer's expected width.
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PLA tolerates under-extrusion better than PETG; fix the cause, do not rely on the material.